The present invention is generally drawn to a system and method for modulating transmitted data to improve the robustness of power-line communications (PLC).
FIG. 1 illustrates a conventional power-line communications system 100.
As shown in the figure, system 100 includes an AC generator 101, a power transmission line 102, multiple power transmission line supports 104, a transmitter 110, communication signal transmission lines 112 and 116 and a receiver 114.
AC generator 101 is connected to power transmission line 102, which is supported by power transmission line supports 104. Transmitter 110 is connected to power transmission line by way of communication signal transmission line 112. Receiver 114 is connected to power transmission line 102 by way of communication signal transmission line 116.
AC generator 101 is operable to generate and distribute AC power through power transmission line 102 to users not shown. Transmitter 110 is operable to transmit a communication signal to power transmission line 102 through communication signal transmission line 112. Receiver 114 is operable to receive communication signal transmitted through power transmission line 102 by way of communications signal transmission line 116.
In operation. AC generator 101 generates electrical power to be distributed to users not shown by way of power transmission line 102, which is supported by multiple power transmission lines supports 104. Transmitter 110 generates a communication signal and transmits the signal by way of signal transmission line 112 to power transmission line 102. The communication signal is propagated by power transmission line 102 and delivered to receiver 114 by communication signal transmission line 116.
Primary impairments that limit communication performance of PLC include frequency selective channel, narrowband interference, and impulsive noise. Frequency selective channel impairments are drawn to signal distortions that are a function of a frequency within a channel in a communication line. Frequency selective channel impairments are based on attributes of the communication medium, e.g., the material of the power line in PLC systems. Narrowband interference is interference within a small portion of the band transmitted by the transmitter. For example, for purposes of discussion, suppose a transmitter is able to transmit in a band from 0 to 500 kHz. In such a transmission scheme, narrowband interference may be interference with the band of 50 kHz to 75 kHz. Impulsive noise may be attributed to electrical devices within the power delivery system. All of these impairments may attenuate and/or delay data transmitted over a power line at different amounts based on the transmission frequency. These inconsistent attenuations and/or delays may cause errors in a received signal in PLC system.
Traditionally, repetition coding is used to improve the robustness of PLC in harsh channel and noise environments, at the price of decreased data rates. As an example IEEE 1901.2, ITU-T G.9903 G3-PLC and ITU-T G9904 PRIME have modes like ROBO (Robust) mode and Super ROBO mode where the bit is repeated either 4 or 6 times. In this manner, even if one, or some, of the repeated bits are corrupted during transmission, there is an increased likelihood that one, or many others, of the repeated bits will be correctly received.
Dual carrier modulation (DCM) has been proposed to combat frequency selective channels in multi-band ultra-wideband (MB-UWB) wireless communication systems using Orthogonal Frequency Division Multiplexing (OFDM) for coherent systems. DCM maps four bits to two different 16-QAM symbols, which are allocated to two sub-bands that are separated by a fixed number of sub-bands. In case one of the two symbols is lost or unrecoverable, it is still possible to recover the four bits from the other symbol. The communication reliability is therefore improved. Using 16-QAM however entails that the system is a coherent system and pilots need to be sent in order to estimate the channel. In particular, the pilots are used to estimate the amplitude and phase of the frequency-selective channel. The channel estimation is then used by the receiver to compensate for amplitude and phase distortion imposed by the channel to subsequently received data. A disadvantage with coherent systems is that the introduction of pilots results in a loss of data rate as compared to a differential system.
As mentioned above, another type of impairment in PLC communication is impulsive noise. Impulsive noise may be attributed to active power components, throughout the power system, that introduce periodic impulsive bursts for noise in the PLC communication system transmission. In PLC, impulsive noise bursts typically span more than one OFDM symbol and have a periodicity equal to half the AC main cycle.
Impulsive noise will be described with reference to FIG. 2.
FIG. 2 is a graph 200 illustrating impulsive noise in a PLC line with respect to a 50 Hz main cycle.
As shown in the figure, graph 200 has a y-axis 202, an x-axis 204 and a function 206.
Y-axis 202 corresponds to the amplitude of an example signal in a PLC line and is measured in volts, whereas x-axis 204 corresponds to time and is measure in ms. Function 206 corresponds to a signal on the PLC line. Function 206 includes a plurality of impulses, samples of which are labeled 208 and 210. The impulses have a period as indicated by double arrow 212, with an ON period indicated by double arrow 214 and an OFF period indicated by double arrow 216. The ON period of the impulses have an average amplitude indicated by double arrow 218, whereas the OFF period of the impulses has an average amplitude indicated by double arrow 220.
Even if DCM is implemented on an impulsive noise channel, there will not be a performance advantage because all of the sub-bands within an OFDM symbol will be corrupted when the impulsive noise occurs. In particular, returning to FIG. 1, suppose that an OFDM symbol were transmitted from transmitter 110 to receiver 114 via transmission line 102. Further, for purposes of discussion, let the noise profile corresponding to function 206, as shown in FIG. 2, be present in transmission line 102. DCM on one OFDM symbol might somewhat address frequency selective channel, narrowband interference associated with transmission line 102. However, if the OFDM symbol were transmitted so as to coincide with an impulse of function 206, for example impulse 208 or 210 as shown in FIG. 2, then the transmitted OFDM symbol would be corrupted as a result of the impulse.
What is needed is a modulation system and method that addresses impairments that limit communication performance of PLC and that does not decrease data rate as much as conventional systems and methods.